Fatal Fences Are Decimating Nature

Scientists have long known that isolation is a killer of nature.

Organisms must be connected to others of their species – to maintain viable populations.

Such connectivity allows species to limit inbreeding, maintain genetic diversity, withstand random demographic fluctuations, and recolonize areas from which they have gone locally extinct.

Many species also rely on annual migrations – for example, to move between distant wintering and breeding areas.

Finally, for many millions of years, species have moved in response to climate change, such as the Ice Ages, shifting to higher or lower latitudes or elevations.  “Move or die” has seemingly been their motto (although a few species could adapt rapidly enough to survive changing climates).

But despite the undisputed importance of movement and population connectivity, habitat fragmentation is continuing apace throughout the world.

And many of the world’s great animal migrations have collapsed or are rapidly declining.


Among the usual suspects in fragmenting our planet – habitat destruction, roads, sprawling urban areas, to name a few – is an old nemesis that many had underestimated.

Scientists are increasingly seeing fences as a big problem, especially for many large mammals, flightless birds, and low-flying species that fail to see fences in their path.

Researcher Penny van Oosterzee reported recently that Africa once supported 14 major large-mammal migrations, but five have failed completely and the remainder all are in trouble.

Fences are a big part of the problem.  For example, Botswana, a Spain-sized nation in southern Africa, today has over 5,000 kilometers of fences just for cattle ranching.

Those fences don’t just stop cattle – they also halt big mammals such as Wildebeest, Hartebeest, and Zebras.

The fences become especially problematic, according to van Oosterzee, when droughts hit and the wildlife would normally migrate to wetter areas.  Following droughts, even quick surveys have revealed hundreds of thousands of dead animals along the fences.

As their prey have declined, predators such as Lions, Leopards, Cheetahs, Hyenas, and African Wild Dogs have also suffered serious population losses.

And Africa is far from alone.  Elsewhere in the world – such as North America, Indochina, Borneo, Australia, and Central Asia – great wildlife migrations have collapsed or are dwindling dramatically.


Fences are just one part of a much broader ‘human footprint’, but their ecological impacts are almost certainly underestimated.

We know that in areas with a heavy human footprint, movements of individual wildlife decline by half to two-thirds, on average.

But how much of that choking effect on wildlife movements is being caused by rapidly expanding fences?  We can say ‘a lot’ – but not much beyond this.

The good news is that international bodies – such as the Convention on Biological Diversity, the Connectivity Conservation Specialist Group, and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services – are increasingly emphasizing the critical importance of maintaining ecological connectivity in the real world.

And with the global explosion of the human populace and our environmental footprint, such efforts are coming not a moment too soon.

As we focus more on connectivity, we can’t ignore the many millions of kilometers of fences on Earth – and their burgeoning impacts on larger animals, whose roles in ecosystems are often profound.

It’s time to take a hard look at fatal fences – and realize how often they are shattering and strangling nature.